1. Field of the Invention
This invention relates to operations on logical volumes in a virtual tape server and more particularly to optimizing operational requests on logical volumes using algorithms.
2. Description of the Related Art
Data Processing systems typically require large amounts of data storage capacity. Portions of the data storage capacity may be needed quickly and may be stored in memory and hard disk drives. Other portions of the data storage capacity may not be required immediately.
As an example, data not immediately required may comprise data that is infrequently accessed. Storage of such data may be in the form of logical volumes of data stored on removable re-writable physical media volumes, such as magnetic tape or optical disk, and the physical media volumes may be written and/or read by means of a data storage drive.
If large amounts of data are to be stored and then accessed on occasion, virtual tape servers (VTS) backed by automated data storage libraries are often employed. Such libraries provide efficient access to large quantities of data stored on physical media volumes which are stored in storage shelves, accessed by one or more users and delivered to data storage drives in the library.
A request by a host data processing system to create or modify a logical volume is issued to a VTS. If the request requires access to a physical media volume that contains the requested logical volume, the VTS instructs its attached library to access the physical media volume from the storage shelf and mount the physical media volume at a desired data storage drive. The logical volume is read to cache storage, which may comprise hard disk drives or other high-speed storage, so that it may be immediately accessed and provided to the host system.
If the request is for a logical volume that is already in cache, or is for a logical volume that will be completely rewritten, a physical media volume access is not required. The host system then reads from or writes to the logical volume in the cache of the VTS. When the logical volume is closed, it remains in cache storage so that it can be immediately re-accessed.
The cache storage is typically limited in capacity, requiring that the updated logical volumes be migrated back in storage so as to free space in the cache storage. Typically a least recently used (LRU) algorithm is employed to migrate logical volumes out of cache storage back to physical media.
Recalling and copying a migrated logical volume requires that the physical media volume containing the migrated logical volume be mounted on a data storage drive. This allows the logical volume to be recalled into cache storage, re-accessed and copied.
Volume mapping is used to create a correlation between the physical capacity of a storage cartridge (stacked volume or physical volume) and the data storage unit size (virtual volumes or logical volumes) of a file or block that is stored on the cartridge. Given the available data storage capacity of a disk, such mapping allows multiple logical volumes to be stored on a single physical volume, hence providing an efficient use of the available storage media. A virtual tape server (VTS) is one device capable of creating and maintaining such mapping among physical volumes and logical volumes.
A typical VTS has a small number of physical drives that can be used for recalling virtual volumes. The VTS has a greater number of virtual drives or devices that provide access to the virtual volumes.
A VTS fills a physical tape by stacking multiple logical volumes. Over time, some of these logical volumes expire and the tape contains less active data. Eventually the physical tape is reclaimed—a process which reads active data from the tape. This active data is scattered on the physical tape requiring that each logical volume be located and read from the tape. At the conclusion of this reclamation process the tape is normally rewound.
Such reclamation is a time consuming process. Typically, the tape is written in a serpentine pattern with multiple passes. Although the location of each logical volume being reclaimed is known, it is a specific location on the tape assuming the serpentine pattern was unwound.
The motion required to read the logical volumes during the reclamation process can be modeled. Due to the time it takes a physical drive to reverse directions, switch from track to track and the effect of high speed locates, the model is non-linear. As such it becomes difficult to predict a best sequence to reduce the time to access logical volumes. In theory the most efficient method can be determined by exhaustively analyzing different sequences of logical volume accesses. In practice this is not feasible as the number of combinations to analyze is impractical.
It would be desirable to use knowledge of the actual location of each logical volume on a tape based on its physical location, along with knowledge of the serpentine pattern, to increase the efficiency and reduce the overall operational time of operational requests. Such operational requests include the recall and reclamation operations of logical volumes on a virtual tape server.